1. Field of the Invention
This invention relates to electronically scanned phased array antennas.
2. Description of the Related Art
The antenna beams of conventional surveillance and communications radar and other communications systems have been mechanically rotated, electrically controlled by phased arrays using phase shifters, or driven by mechanically actuated phase shifters in an attempt to increase range. In the 1950's, phased array technology progressed from mechanical to electronic phase shifters with a phase shifter that used a fixed delay line sandwiched by two frequency mixers (W. H. Huggins, "Generalized Radar Concepts with the use of Array Antennas," Rand Corp., Report RM1854, December 1956). Similar techniques have been used in optical parallel-feed beam forming networks; R. Benjamin, C. D. Iaglanikis and A. J. Seeds, "Optical beam former for phased arrays with independent control of radiated frequency and phase," Electronics Letters, vol. 26, pages 1853-1855, October 1990. In the early 1960's, digitally switched phase shifters used ferrites or diodes.
Although phased array antennas using electronic phase shifters offered distinct advantages over mechanically actuated aperture antennas (i.e. a nearly flat radiating aperture amenable to conformal structures, an electronically scanned beam providing an inertialess steering system, a superior life expectancy, a more effectively synthesized beam pattern for which a number of known algorithms are applicable, and adaptability to hostile environments, including the presence of radar jammers), they were costly and difficult to manufacture for higher frequencies. Phase shifters also experienced temperature-sensitivity, hysteresis and quantization errors, and were limited to the microsecond phase shifting time range (Skolnik, Introduction to Radar Systems, Chapter 8, "The Electronically Directed Phased Array Antenna in Radar," pages 278-342, McGraw-Hill Publishing Co., 1980).
As an alternative to individual phase shifters, simple series-fed frequency-scan arrays have provided beam steering with virtually no phase shifter components except for fixed delay lines. See Skolnik, above, and M. Li, K. Chang, "Novel low-cost beam-steering techniques using microstrip patch antenna arrays fed by dielectric image lines," IEEE Trans. Antennas Propagation, vol. 47, pages 453-457, March 1999. However, in these systems the radiating frequency changes as the beam is scanned, whereas a constant radiating frequency is normally desired.